Styrene-allyl alcohol copolymers are resinous polyols of intermediate molecular weight that are particularly useful for polyesters, fatty ester emulsions, alkyd and uralkyd coatings, and polyurethanes. The most useful styrene-allyl alcohol copolymers known have number average molecular weights of about 800 to 2000 and hydroxyl numbers within the range of about 180-280 mg KOH/g.
Styrene-allyl alcohol copolymers can be made in a batch process by charging a reactor with styrene, allyl alcohol, and a free-radical initiator (such as a peroxide), and heating the mixture at a temperature effective to polymerize the monomers (usually 100.degree.-300.degree. C.). Typical processes are described in U.S. Pat. Nos. 2,630,430, 2,894,938, and 2,940,946, the teachings of which are incorporated herein by reference.
Because of the large reactivity difference between the monomers, a continuous or a semi-batch process is typically used. In the semi-batch process, for example, all of the required allyl alcohol and free-radical initiator are charged to the reactor at the start of the polymerization along with a portion of the styrene to be used (see Example I in U.S. Pat. No. 2,940,946). Most of the styrene is added gradually to the reactor to maintain an excess of allyl alcohol in the reactor. The semi-batch method allows preparation of useful styrene-allyl alcohol copolymers, i.e., copolymers having 15-30 wt. % of recurring units derived from allyl alcohol, average hydroxyl functionalities from about 2 to 6, and hydroxyl numbers in the 180-280 mg KOH/g range.
The conventional semi-batch process gives yields of only about 20-30%, and thus requires recycling of at least about 70 wt. % of the reaction mixture. The yield of polymer product can be increased, but usually only at the expense of making a product having a higher styrene content, lower hydroxyl number, etc., a product that lacks utility for most of the targeted end-use applications.
Still needed in the art is an improved process for making vinyl aromatic/allylic alcohol copolymers, especially styrene-allyl alcohol copolymers. A preferred process would give higher yields of copolymers having desirable molecular weights, hydroxyl numbers, hydroxyl functionalities, and allylic alcohol content. Preferably, the process would be easy to perform, would use conventional equipment, and would be cost-effective.